Behavioural variation at the individual level has been shown to play an important role in animal ecology and evolution. Whereas most studies have focused on subadult or adult subjects, neonates have been relatively neglected, despite studies showing that neonates can exhibit consistent inter‐individual differences during early developmental stages. Steroid hormones, including glucocorticoids (e.g. cortisol) and androgens (e.g. testosterone), play a crucial role in foetal development and maturation and could therefore drive neonate behaviour, although this relationship is poorly understood in wild animal populations. Our study addresses these knowledge gaps by investigating the association between neonate fallow deer Dama dama fawn inter‐individual variability in behavioural response to human handling and hair cortisol and testosterone levels. We found strong evidence that individual neonates display repeatable differences in the way they cope with a risky situation during their first days of life, and showed how these differences are linked to cortisol and testosterone levels accumulated in utero. We showed that, when both steroids are included in the same model, neonates with high cortisol and low testosterone levels coped in a more proactive way with human handling (higher heart rate during handling and shorter latency to leave when released) compared to those with low cortisol and high testosterone levels (lower heart rate and longer latency to leave). These results provide novel insights into the proximate mechanism leading to neonate inter‐individual variation in a wild population of large mammals. A free Plain Language Summary can be found within the Supporting Information of this article.

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Morphometric measurements in wildlife are essential data needed to link behaviour and ecology to life‐history traits. However, acquisition of such data, especially in large vertebrates, is a challenging and limiting aspect in many studies due to associated capture and handling. Ultimately, this prevents testing important hypotheses in wildlife ecology. We demonstrate how to overcome some of these challenges by improving the far‐reaching potential of photogrammetry for wildlife. We validate a methodology using photogrammetry to acquire measurements of complex, curvilinear, morphometric traits (deer antlers) at long range (up to 200 m), providing a methodological foundation for further application to numerous physical attributes and species. We calibrated and validated two different photogrammetric camera systems and applied long‐range photogrammetry to measure a suite of antler features from deer trophies positioned analogous to free‐living, wild deer. We also developed, validated and described an efficient correction protocol to overcome the complexity of three‐dimensional antler features. This complexity previously biased the acquisition of absolute measurements. Our measurement estimates were characterized by mean errors lower than 5% when compared to true sizes, with our correction protocol for antlers particularly effective and validated with independent antler sizes. Importantly, the estimated size rankings based on the true sizes were identical to the rankings estimated by photogrammetry in all cases. We measured antlers in free‐living, wild fallow deer as a field validation and demonstration, showing how we were able to collect physical trait measurements of 230 antlers without capture. This non‐invasive, remote and fast technique can provide valuable and accurate data allowing researchers to address new hypotheses by linking life‐history traits to wildlife ecology, behaviour and management.